Liquid crystals are soft matter systems in which strongly anisometric liquid crystal molecules display long-range orientational order, but possess partial or no long-range positional order. The anisotropic optical properties of liquid crystals are also the basis for their large birefringence. In addition, liquid crystal molecules possess anisotropic electric and magnetic susceptibilities and the orientation of liquid crystals can be changed by external stimuli, such as electric and magnetic fields. Therefore, these advanced materials have been extensively utilized in variety of display applications.
The liquid crystal/polymer composites of various types represent an important and broad class of three-dimensional structures utilized at the forefront of flexible liquid crystal display science and technology. Polymer dispersed liquid crystals and polymer stabilized liquid crystal systems may be given as the classical examples for the use of liquid crystal-polymer composite structures in display applications. In liquid crystal-polymer composites, the polymer component serves a number of critical functions: provides mechanical support (ruggedness); determines the thermomechanical stability of the composite; protects the device or fiber functionality from environment; helps to distribute the applied pressure by acting as a stress transfer medium (self-sustaining); provides durability, interlaminar toughness and shear/compressive/transverse strengths to the system in general, and maintains the cell gap of the device. Therefore, synthetically produced polymeric materials have been the key component toward producing self-sustaining and self-adhering flexible display prototypes.
The encapsulation of liquid crystal materials into polymer morphology via phase separation techniques such as polymerization induced phase separation (PIPS); thermally induced phase separation (TIPS); solvent induced phase separation (SIPS); UV Intensity gradient induced anisotropic phase-separation; and electric field induced phase separation—has been used to prepare light modulating devices. [J. L. West, Mol. Cryst. Liq. Cryst. 1988, 157, 427] Given that chemical and physical properties and the morphology of liquid crystal-polymer composites can be tailored for variety of applications, they have been utilized in sophisticated optical and electronic devices, such as in light shutters, bistable displays, switchable windows, portable electronics, and beam steering devices. In addition, liquid crystal materials can be confined to porous matrices, capillary tubes, cavities, and prefabricated inclusions by an infiltration (or permeation) method. The most often used matrices are controlled-pore glasses [Z. Kutnjak, et al., Fluid Phase Equilibria 2004, 222-223, 275], aerogels [N. A. Clark, et al., Phys. Rev. Lett., 1993, 71, 3505], and aerosil particles [G. S. Iannacchione et al., Phys. Rev. E 1994, Vol. 50, No. 6, pp 4780-4795 and G. S. Iannacchione et al., Phys. Rev. E 1997, Vol. 56, No. 1, pp. 554-561]. In order to create optically responsive textiles and to enhance the overall flexibility of systems, researchers have produced electro-optical devices on fabrics utilizing liquid crystal-polymer composites, as well as other existing display technologies. A few examples include a reflective cholesteric LCD fabricated by sequential coating of functional layers on fabric [A. Khan, et al., SID Int. Symp. Deg. Tech. Pap. 2006, 37, 1728], light emitting diode (LED) illuminated optical fibers woven into fabrics [V. Koncar, Optics and Photonics News, 2005, 16, 40], and organic-light-emitting-diode (OLED) coated fibers [K. Yase, et al., SID Int. Symp. Dig,. Tech. Pap., 2006, 37, 1870]. Although these prototypes provide some degree of flexibility, they all negatively impact the physical characteristics of textiles, such as full flexibility and breathability. Conversely, incorporation of liquid crystal molecules directly into fibers may be a route to combine fiber/textile and optoelectronic properties of liquid crystals, wherein an electrospinning method was utilized in order to create ultrafine—and at the same time—optically responsive electro-optical composite liquid crystal fibers.
Polymeric fibers can be formed by a range of methods including melt spinning, melt blowing, wet spinning, gel spinning, dry-jet wet spinning, dry spinning and electrospinning. The method of fiber formation is chosen based on the properties of the polymer and the dimensions and physical properties desired in the final fibers. Secondary components including a second polymer or small molecule components can be incorporated into the fibers during the spinning process either as mixtures with the main polymer or in separate domains by methods known to those versed in the art of fiber spinning. Methods for forming bi-component fibers include coaxial spinning or extrusion and spontaneous phase separation during the spinning process.
The electrospinning method has been employed extensively to produce engineered fibers ranging from polymers to ceramics for various applications, including energy storage, tissue engineering, drug delivery, chemical and biological sensors, membranes, and filters. [D. H. Reneker, et al., Polymer 2008, 49, 2387; D. Li et al., Journal of Membrane Science, 2006, 279, 354, and D. Li, et al., Polymer, 2007, 48, 6340.] Electrospinning from solutions of polymers mixed with small molecules, such as isotropic liquids or liquid crystal materials, have been explored in recent times. For instance, the coaxial electrospinning of nematic liquid crystals and a poly(vinylpyrrolidone) (PVP)/TiO2 sheath was recently reported by Lagerwall et al. [J. P. F. Lagerwall, et al., Chem. Commun., 2008, 5420] Similarly, liquid crystalline polysiloxane with cholesterol side chain and small molecule liquid crystal was electrospun by Wu et al. [Y. Wu, et al., Colloid. Polym. Sci, 2008, 286, 897] to prepare liquid crystal fibers to create high performance materials. Electrospinning of liquid crystalline elastomers has also been studied for potential use as mechanical actuators because of the anisotropic physical properties associated with this class of materials.